1. Field of the Invention
The present invention relates to an optical component molding apparatus for molding optical components and, more particularly, to an optical component molding apparatus for molding optical components each of which has a small size (an outer diameter of 2 mm to 12 mm) and high precision (surface roughness Ra of 20 nm or less).
2. Description of Related Art
Recently, optical components such as objective lens are formed in a manner of injection molding using a thermoplastic resin (refer to, for example, Patent Documents 1 and 2). Since, by molding optical components using a plastic, products of uniform configuration can be produced quickly, the molding is suitable for mass production thereof. Optical instruments utilizing the plastic lens have a tendency toward miniaturization and higher performance from year to year. Accompanying this, requests for miniaturization and higher precision of the plastic lens are also increasing.
Further, as a technique for mass production of optical components, there has been studied a so-called multi-cavity molding technique for molding a plurality of molded products in a single mold-clamping and resin-injecting operation. A mold used for the multi-cavity molding has a single sprue centrally provided in a fixed mold and a plurality of runners arranged around the sprue. That is, lens transfer sections are arranged around the sprues. In this multi-cavity molding technique, how to increase the number of molded products while reducing variations in transferability to each transfer section has a large influence on commercial profit.
[Patent Document 1] Japanese Published Unexamined Patent Application No. 11(1999)-42685
[Patent Document 2] Japanese Published Unexamined Patent Application No. 2001-272501
However, the above molding of plural optical components having a small size and high precision would be problematic in the following points. To be concrete, it is known in the injection molding of optical components that each shape of a gate and a runner has an influence on transferability for molding and stability of molded products. It is further noted that the shape of a sprue as well as the shapes of gate and runner exerts an influence on the transferability for molding and the stability of molded products.
For instance, when the thickness of the gate (hereinafter, “gate thickness”) is small, a flow channel for resin is sharply narrowed down at the gate. To ensure good transferability, therefore, the resin has to be injected at high speed. This may cause large shear heating. This results in a high temperature condition of the resin, leading to a deterioration in the quality of the resin. Also, stress likely concentrates on near the gate and thus birefringence would appear significantly. Further, such molded products are likely to become deformed during mold releasing and lack product stability.
If the diameter of an outlet of the sprue is small, the temperature of the resin is decreased fast. This makes it insufficient to inject resin into the transfer section and consequently difficult to achieve good transferability. Further, other defects such as a demolding failure of molded products, remaining in the fixed mold, and a mold-releasing failure would inevitably arise. If the sprue has an outlet of a large diameter, in contrast, it would take a long time to cool and solidify the resin in the sprue. In other words, the cooling time (duration) is determined depending on the time needed for solidifying the resin in the sprue. This makes it difficult to shorten a producing cycle. The injection pressure and injection speed of resin would be unstable, leading to instability of molded products.